Many pathogens use the intestinal mucosa as the primary site of entry. The intestinal mucosa must induce a rapid and strong immune response to defend against such pathogenic invasion, yet an excessive immune response to commensal flora leads to chronic inflammation like inflammatory bowel disease. While it is evident that mucosal antigen-presenting cells (APCs) play a key role in orchestrating the effector response to pathogens, it is unclear how APCs maintain homeostasis with commensals. At the initiation of an immune response, mucosal APCs sense pathogens by using toll- like receptors (TLRs) that in turn signal through the adapter molecules MyD88 (myeloid differentiation factor 88) or TRIF (Toll/interleukin-1 receptor domain-containing adapter inducing IFN-?). Most research has focused on the MyD88 pathway because it is used by almost all TLRs and it leads to a robust NF-?B response. However, strong NF-?B activation may also destroy host tissues. By contrast, the TRIF pathway is only induced by TLR4 or TLR3 and supports regulatory responses through induction of IFN?. Therefore, there must be unique roles of TRIF pathway in intestinal immune regulations, but how this pathway contributes to host defense against intestinal bacterial infection has not been studied. Our ultimate goal is to protect the intestinal mucosa from infection and chronic inflammation through the future development of vaccines and immunotherapies. But fundamental research is needed to understand specific aspects of TLR signaling in the intestinal mucosa to achieve this goal. The objective of this application is to understand how APCs regulate intestinal immune responses through TRIF signaling. We wish to test the hypothesis that TRIF signaling in APCs regulates protective immune responses in the intestine. Using Yersinia enterocolitica as a model enteric pathogen, we found that TRIF-deficient mice had a significant defect in macrophage bactericidal activity. They succumb to infection due to systemic Y. enterocolitica dissemination. These findings lead us to examine how TRIF signaling in APCs regulates intestinal immune responses during bacterial infection. Once the mechanism of TRIF-mediated immune defense is elucidated, this signaling can be pharmacologically manipulated leading to innovative approaches in the management of inflammatory and infectious diseases affecting the intestine. Targeting TRIF is advantageous because a clinically tested TLR3 ligand is available.